SIDEBANDS FREQUENCY 1

[bk33]

hi all
can somone please tell how should i better identify sideband frequencies. example if i have a 34 Hz, should i be looking at 17hz.
Thankyou

 

[Hatch]

The 17 Hertz frequency you quoted is not a sideband frequency it is a harmonic i.e. 1X, 2X etc or a sub harmonic 1/2X (0.5 times).

In vibration analysis a motor turning a fan at 30 Hertz (1800rpm) will generally show unbalance at the fundamental frequency of 30 Hertz or 1X (1 times operating speed). Misalignment will show up as 2X and other problems will show up at various multiples (this is a simplification).

I am not sure if that answers you question on sidebands. If you really mean sidebands, then it is also possible that a bent shaft could modulate the fan blade pass frequency (fan speed x number of blades). Thus if there were 7 blades and a direct coupling of the motor to the fan the blade pass frequency would be 210 Hertz. The bent shaft could cause sideband frequencies of 180 Hertz (210-30) and 240 Hertz (210+30). The modulation effect also occurrs in many other situations, particularly with gearboxes.

 

[bk33]

thankyou Hatch
yes you right the 17 Hz is a harmonic
what really puzzeld me is a machine that i was monitoring which is a gear box running at 3500rpm with 7.12 to Gear Ratio. And i had 17 Hz 34Hz 51 hz and i was expecting a 58Hz

 

[noise]

Your tone at 17 Hz may be twice the low shaft speed of 8.19 Hz, depending on your frequency resolution. The low shaft speed is 3500/(60x7.12) = 8.19 Hz. You could possibly find side bands that modulate the gear mesh frequency (rotation rate x number of teeth). For instance 40 teeth on the high speed shaft would have a gear mesh freq. of 3500x40/60 = 2333.33 Hz. Side bands at 58.3 Hz would indicate a modulation of the gear mesh from the high speed shaft, modulation at 8.19 Hz would indicate modulation by the low speed shaft.

 

[Hatch]

Yes Noise (Mechanical) has raised some good points. However, you have not said if the gearbox is a single or double reduction gearbox. The intermediate shafts and gears all have a gear mesh frequency and a shaft speed. It is quite possible that you have a problem with one of those components. So you have to calculate each of the shaft frequencies (input-intermediate-output) and each of the gearmesh frequencies and compare them to your measured data.

A word of warning, it can be difficult to get the number of gear teeth data etc for a gearbox.

 

[bk33]

hi Hatch,Noise thanks for your shared information
i spent my day yesterday getting the data of that unit ..
the gear ratio of is 7.12 to 1

25teeth on the gear on the input shaft

the input speed is 3500 cpm/rpm / 58Hz

178 teeth on the gear on the output shaft, having the GMF
about 1456 Hz
is it possible to say that 17 Hz and harmonic with the side bands is a strong candidate for a gear prob.

 

[Hatch]

You are getting closer to a solution, but there is still a lot of info to shift through.

1. Gear mesh appears to be ok, 17Hertz seems to be related to the output shaft (2X). Therefore check for coupling misalignment on the output shaft, and the coupling itself. Check output shaft bearings. Check the output shaft bearing frequencies.
2. Look at the gearmesh frequency and zoom in (using FFT). Are there any + or - sideband frequencies of 17 Hertz i.e. 1458 + 17 or 1458 -17 Hertz.
3. Is there anything on the driven machine which operates at 17 Hertz?
4. Is the vibration higher on the output shaft bearing or on the input shaft bearing. What direction is worst?

Vibration analyis requires a substantial amount of data analysis. Without the historical data on the gearbox and inspection, I can only guess that your input shaft is ok but you need to check the output shaft, output shaft bearings and the coupling.

 

[bk33]

Hi Hatch.Noise
zooming in on the fft the only sideband freq nearrer to the
(1458+17 1458-17) i found 1485hz and 1440hz but in very low Magnitude.
the 17Hz Harmonics i still puzzling me even tough it seems to be related to the output shaft
vibration seems to be higher in the input shaft.
another interesting point you made is the history of the unit which i dont have this the first time im working on this unit .
thanks

 

[Hatch]

One more thing:

Just remember that one of the two output shaft bearings is probably located on the input side of the gearbox. Is the amplitude at 17 Hertz frequency higher at that point? Usually the bearing housing sticks out so you can take a measurement, but there is a cover over the bearing.

Also zoom in to the 17 Hertz frequency. Is it exactly at 17 Hertz or some slightly different freq i.e. 16.38 Hertz.

Shut the unit down and record a coastdown (via tape). Play it back at 1/2 speed and see if you can determine if the 17 Hertz frequency changes with the shaft speed, or is a fixed frequency.

And lastly, if you have a photo-trigger, use it to do a synchronous average (more advanced fft) of the vibration with the input shaft as the trigger and see which frequencies show up.

We have not talked direction, but is the 17 Hertz frequency radial or axial or both?


Again it gets complicated but it is always interesting.